1. Field of the Invention
This invention relates to an image processing apparatus, and more particularly to an image processing apparatus suitable for reducing block distortions occurring in the decoded image in an image encoding/decoding system that compression-codes images into a small amount of data and reproduces the images by decoding the code string obtained through compression coding.
2. Description of the Related Art
Various schemes, including motion compensation, discrete cosine transform, subband coding, and pyramid coding schemes, and combinations of these schemes have been developed as the techniques for compression-coding images into a small amount of data for transmission or storage in the field of systems that transmit and store images, including videophones, teleconference systems, personal digital assistants, digital video disk systems, and digital TV broadcasting systems. ISO MPEG1, MPEG2, ITU-T H. 261 and H. 262 have been prescribed as the international standard schemes for motion-picture compression coding. Each of these schemes is a compression coding scheme that is a combination of motion compensation adaptive prediction and discrete cosine transform. The details of these have been given in, for example, reference 1 (Hiroshi Yasuda, "The Standards for Multimedia Coding", Maruzen, June, 1991).
With a conventional motion picture coding apparatus using motion compensation adaptive prediction and discrete cosine transform, the input picture signal is divided into a plurality of blocks and then each block undergoes motion compensation adaptive prediction. Specifically, the motion vector between the input picture signal and the reference picture signal of the preceding frame is sensed and a prediction signal is created by performing motion compensation on the reference picture signal using the motion vector. In this case, the prediction mode for the more suitable one of the motion compensation prediction signal and intra-frame encoding (prediction signal=0) in which the input picture signal is used directly in coding is selected and a prediction signal for the prediction mode is produced.
Next, the prediction signal is subtracted from the input picture signal to produce a prediction error signal. The prediction error signal undergoes discrete cosine transform (DCT) in blocks of a specific size. The DCT coefficients obtained from the discrete cosine transform are quantized. The quantized DCT coefficients are subjected to variable-length coding and then are multiplexed with coded motion vector information and thereafter the resulting signal is outputted as a code string. On the other hand, the code string is dequantized and then is subjected to inverse discrete cosine transform. The prediction error signal restored by inverse discrete cosine transform is added to the adaptive prediction signal to produce a local decoded signal. The local decoded signal is stored in a frame memory as a reference picture signal.
The code string that has been transferred from the motion picture encoding apparatus and has been stored is inputted to the motion picture decoding apparatus, which then separates the code string into the quantized DCT coefficients and the motion vector information. The quantized DCT coefficients are subjected to variable-length decoding, dequantization, and inverse discrete cosine transform and is thereby reconstructed into a prediction error signal. The motion vector information undergoes variable-length decoding and thereafter a motion compensation prediction process. In the motion compensation prediction process, motion compensation is performed on the reference picture signal of the preceding frame stored in the frame memory to produce a prediction signal. Then, the prediction signal is added to the prediction error signal. This addition produces a picture signal. The reproduced picture signal is outputted outside the decoding apparatus and is also stored in the frame memory as a reference picture signal.
The aforementioned conventional motion-picture encoding/decoding apparatus has the following problems.
Since block coding using orthogonal transform, such as discrete cosine transform, carries out orthogonal transform block by block, it is difficult to sense changes in the signal between blocks. In addition, when coding is done at a low bit rate, the quantization width of the quantizer that quantizes the DCT coefficients (orthogonal transform coefficients) must be made large, making the alternating-current components (high frequency components) more liable to be lost. As a result, smooth changes at the boundary between blocks cannot be represented and the boundary between blocks appears noticeably on the screen in the form of lattice block distortion.
An alternating-current (AC) component prediction scheme using average value has been proposed as a method of predicting the AC components lost in quantization (reference 2: Watanabe and Ohzeki, "Evaluation of an Alternating-Current Component Prediction Scheme Using Average Value," the 1989 picture coding symposium (PCSJ89), 2-2, 1989). Additionally, several other AC prediction schemes have been proposed.
The conventional AC component prediction schemes, however, were based on the assumption that the AC component prediction process is carried out before discrete cosine transform in the coding process. The prediction schemes were not used for removal of block distortions occurred in the decoded pictures. Even if the conventional AC prediction schemes were applied directly to the decoded pictures, the predictable AC components would be limited to only the low-frequency AC components close to a direct current (DC) and the high-frequency AC components in the decoded pictures would be removed.
A method of reducing block distortions has been proposed (reference 3: Izawa, Watanabe, and Takizawa, "Band Preserving Block Distortion Removing Filter in Picture Coding," the 1989 picture coding symposium (PCSJ89), 4-4, 1989). This method is such that the decoded picture obtained by decoding the code string subjected to discrete cosine transform and quantization in blocks of n.times.n pixels is forced to undergo discrete cosine transform again in large blocks of 2n.times.2n and the resulting DCT coefficient string is decoded, with the band being limited to almost the same coefficients as the DCT coefficients transferred during coding. This makes it possible to estimate changes in the boundary between blocks, alleviating block distortions. This method, however, requires discrete cosine transform in blocks, each twice as large as the original one in length and breadth, on the decoding side and the encoding side, which makes the decoding process more complex.
Furthermore, quantization causes quantization distortion called mosquito noise around the edge portion, degrading the picture quality. The distortion has particularly a great effect on a flat domain adjacent to the edge portion. A nonrecursive 5.times.5 adaptive smoothing filter has been proposed as a method of reducing the distortion (reference 4: Kato and Ohkubo, "Improvement of High-Efficiency Coding Picture Quality by Post Filtering", Proceedings 1989 Electric Information Communications Society Autumn National Meeting, Vol. 6, D-3, D-63). Since the method eliminates the high-frequency AC components by applying the filter to the entire screen, the edge portion is blurred a little.
As described above, the conventional motion-picture encoding/decoding apparatus has the problem that block distortions occur at the boundary between blocks as a result of carrying out the processing block by block and lattice distortions appear conspicuously. The conventional method of reducing the distortions is complex in processing. Furthermore, because the method of simply predicting the lost AC components removes the AC components from the blocks originally having the high-frequency AC components, it has an adverse effect on the picture quality.